WO2013049378A2

WO2013049378A2 - Methods for treatment and use of produced water

Info

Publication number: WO2013049378A2
Application number: PCT/US2012/057605
Authority: WO
Inventors: Rafique Janjua; Robert PRIETO
Original assignee: Fluor Technologies Corp
Current assignee: Fluor Technologies Corp
Priority date: 2011-09-27
Filing date: 2012-09-27
Publication date: 2013-04-04
Anticipated expiration: 2014-03-27
Also published as: US20130075098A1; CA2849984A1; WO2013049378A4; WO2013049378A3

Abstract

Produced water is treated by raising the pH to a level that significantly increases silica solubility and breaks emulsions. So treated water is then de-oiled, filtered, and subjected to ion exchange chromatography to reduce water hardness prior to feeding into a steam generator to form an intermediate quality steam. If desired, the intermediate quality steam is directly used in SAGD, or separated into a high quality steam and condensate, which is further treated to obtain additional water that can then be used in the steam generator.

Description

METHODS FOR TREATMENT AND USE OF PRODUCED WATER

[0001] This application claims the benefit of priority to copending U.S. provisional applications with serial numbers 61/539,883, filed September 27, 2011, and 61/621,145, filed April 6, 2012.

Field of the Invention

[0002] The field of the invention is methods and use of treated produced water, especially as it relates to treatment and use for enhanced oil recovery (EOR) and steam assisted gravity drainage (SAGD).

Background

[0003] Enhanced Oil Recovery (EOR) of heavy thick oil or Bitumen is often done by injecting steam into a formation, resulting in a combination of condensed steam and oil and/or melted bitumen that is later separated into the hydrocarbon product and "produced water". For example, steam-based tar sands EOR, also known as steam assisted gravity drain

(SAGD), requires about three barrels of water equivalent of steam to produce one barrel of liquid bitumen product. Such relatively high quantities of water demand can present significant challenges, especially where recovery and recycling of the water is required.

[0004] Among other difficulties, produced water from SAGD has frequently high quantities of emulsions of liquid bitumen and dissolved solids, suspended solids, and free or floating oil, which requires significant treatment for production of usable water. Currently known treatment methods include de-oiling, filtration for suspended solids removal, warm/hot lime softening for hardness or silica removal, or primary and secondary weak acid cation ion exchange. Typical examples of known treatment methods include those described in U.S. Pat. No. 8,047,287 and WO 2012/122207 where one or more membranes are used to remove silica and/or oil from produced water. Similarly, as described in U.S. Pat. App. No. 2008/0135478, produced water is treated by a series of steps that include degasification, chemical softening, filtration, ion exchange, and reverse osmosis. In yet other known methods, flotation processes are used as taught in U.S. Pat. App. No. 2009/0014368 while WO 2005/054746 teaches processing of de-oiled produced water through a high pH/high pressure evaporator to generate a vapor suitable for SAGD to so avoid use of once through steam generators that would require extensive chemical treatment. [0005] In still further known methods, as described in WO 2012/048217 an evaporative process using a crystallizer is disclosed. Yet further known methods are described in WO 2007/051 167, WO 2009/006575, and WO 2012/024764. These and all other extrinsic materials discussed herein are incorporated by reference in their entirety. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0006] Unfortunately, de-oiling of produced water containing tight emulsions using commonly known industry practices (e.g., hydro cyclone, induced or dissolved gas flotation, etc.) are often not effective. Consequently, most of the oily emulsion will pass through to the downstream units, which tends to undermine the effectiveness of filtration, lime softening, or ion exchange systems. As a result, poor quality water is injected into the high pressure boiler for steam generation and leads in many cases to frequent and very expensive break downs. Alternatively, high gravity force centrifuges (e.g., exceeding 3,000 xg force) can be used to break an emulsion. However, such approach generally requires a large number of centrifuges, which is often cost prohibitive and consumes significant plot space. To circumvent such difficulties, emulsions can be acidified to a pH of less than 3 as low pH solutions can break the emulsion. However, such low pH will require neutralization or alkalinization prior to further treatment, and require large amounts of acid and caustic materials and special metallurgy. Moreover, handling of large amounts of acid and base will increase safety hazards.

[0007] Compounding the above difficulties in current SAGD reservoir operations is the use of high quality steam (approaching 100%) from which silica and other cations have been removed to a large degree. The removal of silica has been previously driven by process consideration and the removed silica has become a high volume hazardous waste product of SAGD operations that must be properly disposed of. Moreover, many operators are generally opposed to the use of high silica water in SAGD due to concerns over silica deposition and precipitation in high-value equipment (e.g., once through steam generators and steam separators). Additionally, many operators have raised concerns with respect to reduced produced water flows as a result of silica deposition in the interstitial region between horizontal injector and extraction wells, even though these concerns were not subject to detailed assessment (presumably because of the concerns associated with high- value equipment protection). As a consequence, and due to the relatively high silica content of most produced water, operators tend to rely on expensive processing equipment for water treatment prior to steam generation, which presents a significant economic impact. Worse yet, steam generation and particularly high-quality steam generation is energy intensive and further exacerbates the economic challenges with continuous SAGD.

[0008] Thus, there is still a need for processes, devices, and methods that break down or remove emulsions to enhance filtration processes, reduce or even eliminate the need for hot/warm lime softening, and improve methods of steam use in EOR and especially in SAGD.

Summary of The Invention

[0009] The inventive subject matter provides devices, systems and methods in which produced water is processed by (preferably chemically) breaking emulsions such that the produced water can be de -oiled and filtered in simple processes, and by elevation of the pH to a degree that substantially increases silica solubility. Once de-oiled, filtered, and alkalinized, the treated water is fed to a steam generator, most preferably a once through steam generator (OTSG), to produce an intermediate quality steam. The intermediate quality steam is then either directly used for injection for SAGD, or separated in a condensate separator to thereby produce high quality steam and a condensate that can then be processed to produce a purified water product suitable for additional steam generation.

[0010] In one aspect of the inventive subject matter, the inventor contemplates a method of processing produced water that includes a step of providing a quantity of produced water, and a further step of de-oiling and removing solids from the produced water to thereby form treated water. In another step, divalent cations are removed from the treated water using an ion exchange resin (e.g., a weak acid cation ion exchange resin) to thereby produce softened water, and in a still further step, alkalinity of the softened water is increased to form alkalinized water. In yet another step, an intermediate quality steam product is formed from the alkalinized water, and at least a portion of the intermediate quality steam product is used for injection into a formation.

[0011] In some aspects, it is preferred that the step of using a portion of the intermediate quality steam product comprises separation of the intermediate quality steam product into a condensate and a high quality steam product, and a further step of injection of the high quality product into the formation. Here, the condensate may be further processed to so obtain additional water, which may be uses in forming the intermediate quality steam product. The condensate may then be processed in a brine concentrator. In other aspects, it is preferred that the step of using at least a portion of the intermediate quality steam product comprises use of substantially all of the intermediate quality steam product for injection into the formation.

[0012] Regardless of the manner of use of the intermediate quality steam (which typically comprises between 10 and 30% condensate), it is generally preferred that the intermediate quality steam is formed in a once-through steam generator, and that the alkalinized water has a pH of at least pH 10. It is further especially preferred that all process steps are performed without an active step of removing silicate from the produced water, the treated water, the alkalinized water, and/or the softened water (e.g., silicate removal via lime softening).

Indeed, it is noted that the methods presented herein are even suitable for adding previously isolated silica to the produced water, the treated water, the softened water, and/or the alkalinized water.

[0013] Viewed from a different perspective, the inventor also contemplates a method of processing produced water that includes a step of providing produced water comprising silica, and de -oiling and removing solids from the produced water. In another step, divalent cations are removed from the produced water using an ion exchange resin, and in yet another step, a base is added to the produced water to thereby form alkalinized water, wherein the base is added in an amount that increases solubility of the silica in the alkalinized water at least 25% as compared to the produced water.

[0014] In especially preferred methods, the base is an alkaline metal hydroxide or an alkaline earth metal hydroxide, and is added in an amount to achieve a pH of at least 10 in the alkalinized water. While not limiting to the inventive subject matter, it is preferred that the step of de-oiling comprises a step of chemically breaking an emulsion in the produced water. If desired, an additional step may include forming an intermediate quality steam product from the alkalinized water, and yet another step of using at least a portion of the intermediate quality steam product for injection into a formation.

[0015] Similarly, the inventor also contemplates a method of processing produced water that includes a step of providing produced water comprising silica. In another step, an emulsion is chemically broken in the produced water, and divalent cations are removed from the produced water using an ion exchange resin. In still further contemplated steps, a base is added to the produced water to thereby form alkalinized water, and an intermediate quality steam product is formed from the alkalinized water without a step of removing silica from the produced water.

[0016] Most preferably, the base is added in an amount to achieve a pH of at least 10 in the alkalinized water, and where desired, previously isolated silica may be added to the produced water, the alkalinized water, and/or the softened treated water. Contemplated methods may also include a further step of using at least a portion of the intermediate quality steam product for injection into a formation.

[0017] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments.

Detailed Description

[0018] The inventor has now discovered that produced water can be treated for subsequent use in a conceptually simple and effective manner avoiding various expensive processing steps. Most preferably, produced water is treated with one or more chemicals to a degree that is effective to break emulsions that are present in the produced water. So treated water is then de-oiled using conventional separation, and most preferably by one or more separation processes that do not require centrifugation or other mechanically complex devices.

Therefore, downstream processes (e.g., filtration, ion exchange processes) that would otherwise be adversely affected by emulsions are now easily implemented. Once de-oiled and solids/precipitates have been removed, the water is alkalinized to a degree that is effective to reduce, or even entirely eliminate the need for silica removal, which in turn allows use of the alkalinized, de-oiled, and filtered water in downstream processes without further processing.

[0019] For example, the de-oiled, and filtered water can now be softened using an ion exchange resin (most preferably a weak acid ion exchange resin), which advantageously avoids use of hot/warm lime softening, and the alkalinized, de-oiled, and filtered water can be directly used in a steam generation process (and most preferably in a once-through steam generator, with or without ion exchange without softening).

[0020] In yet further preferred aspects, steam generation of the alkalinized, de-oiled, and filtered water will provide an intermediate quality steam product (typically about 70% steam) that can be directly used for injection into a formation, or that can be processed in a condensate separator to generate high-quality steam and a condensate suitable for further processing (preferably using an evaporator) to thereby produce additional water for the steam generator or other purpose. Thus, it should be appreciated that the silica content of the water that is being fed to the steam generator is in many instances the same (or some instances even higher) than the silica content of produced water entering the process.

[0021] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventive elements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements A, B, and C, and a second embodiment comprises elements B and D, then the inventive subject matter is also considered to include other remaining combinations of A, B, C, or D, even if not explicitly disclosed.

[0022] In one preferred aspect of the inventive subject matter, a method of processing produced water is contemplated that includes a step of providing a quantity of produced water, and another step of de-oiling and removing solids from the produced water to thereby form treated water. After removal of divalent ions (preferably using a weak acid ion exchange resin) alkalinity of the treated softened water is increased to so form alkalinized water. It should be noted that the step of de-oiling most preferably uses one or more chemical agents that break emulsions, and all known chemical emulsion breakers are deemed suitable for use herein.

[0023] With respect to the produced water it is generally contemplated that all known manners of providing produced water are deemed suitable for use herein. Thus, suitable produced water may be derived from steam-assisted hydrocarbon recovery, from on-shore and off-shore oil and gas production, and various other sources in hydrocarbon processing.

Consequently, produced water is typically separated from a mixture of the produced water and a hydrocarbon product, and in most cases such separation is via gravity/phase separation is separators well known in the art. However, in alternative aspects of the inventive subject matter, previously stored or otherwise sequestered produced water may also be employed in conjunction with the teachings presented herein.

[0024] Regardless of the source of the produced water, and regardless of the order of various other possible treatment steps (e.g., de-oiling, softening, filtration, etc.) it is generally preferred that the produced water is alkalinized such that the pH of the alkalinized water is at least 8.5, more typically between 8.5 and 9.5, even more typically between 9.5 and 10.5, and most typically between 10 and 1 1 (and in some cases even higher). Viewed from another perspective, it is preferred that the pH of the alkalinized water is higher than the pH of the produced water prior to alkalinization, and typically at least 0.5 pH units, more typically at least 1.0 pH units, and most typically more than 1.5 pH units. Thus, the pH is raised in the produced water to a level such that solubility of silica in the alkalinized water is increased over the solubility of silica in the produced water, in most cases at least 50% (at standard temperature 20 °C and atmospheric pressure), more preferably at least 100%, and most preferably at least 200%. Viewed from a different perspective, it is generally preferred to raise the pH such that silica removal as practiced in heretofore known processes is no longer required. Indeed, it is contemplated that the pH can be raised to a level that allows adding previously isolated silica to the produced water (the alkalinized water, and/or the softened treated water) before the water is fed to the steam generator.

[0025] As should be readily appreciated, there are numerous methods of alkalinization known in the art, and all of the known methods are deemed suitable for use herein. However, in especially preferred methods, alkalinization is performed by adding a strong base to the produced water in an amount sufficient to raise the pH to the desired level. For example, suitable bases include hydroxides of alkaline metals or earth alkaline metals, which may be provided in solid or liquid form.

[0026] It should further be appreciated that the produced water will still contain significant quantities of hydrocarbons, most typically in form of emulsions and even tight emulsions due to the SAGD process. Thus, de-oiling will also typically include a step of breaking an emulsion and removal of the oil from the broken emulsion. There are numerous manners of emulsion breaking known in the art, and all of those are contemplated suitable for use herein. However, in especially preferred aspects, one or more chemical agents are used to break emulsions. Once more, there are numerous chemical agents for breaking emulsions known in the art, and all of those are appropriate for use. Depending on the particular type and oil content in the emulsion of the produced water, it should be noted that the water may be subject to a resting period after breaking of the emulsion (e.g., in a surge tank or other holding vessel) to promote or allow for phase separation, or may be directly processed in a de-oiling process. It should be noted that the step of de -oiling is typically performed in conventional manner. Thus, de-oiling may be done in an API-type separator, in a demulsifier, in a skim tank, a centrifugal or hydrocyclone separator, or using a plate- or enhanced-coalescers.

Moreover, de-oiling may also be performed using various absorption media well known in the art.

[0027] Likewise, it should be noted that removal of solid phase (e.g., sand or other minerals, or precipitates) may be performed as part of de-oiling operation and so form sludge, and/or may be separately performed by gravity separation or filtration using settling or filtration devices well known in the art.

[0028] Consequently, it should be noted that the downstream processes (e.g., filtration) are also enhanced as the liquid is less prone to foul filters and/or filtration materials. Moreover, due to the increased silica solubility attributable to the high pH, hot/warm lime softening can be reduced, and even more typically entirely eliminated. If hardness remains a concern, it is noted that water softening of the (de -oiled and/or alkalinized) water can be performed using ion exchange chromatography to so remove divalent cations that contribute to water hardness and scaling in equipment. There are numerous suitable ion exchange resins known in the art to remove such ions, and all of those are deemed suitable for use herein. Especially preferred resins include weak acid cation exchange resins.

[0029] Consequently, it should be recognized that contemplated processes and methods will significantly reduce scaling of steam generators, require less equipment and chemicals to process, and produce less solid waste. Moreover, it should be noted that plants using contemplated methods and processes will require a smaller real-estate footprint, require less water to produce an equivalent amount of product, and will simplify operation or

maintenance.

[0030] Viewed from another perspective, the inventor also contemplates a method of processing produced water comprising silica. In such methods, a base (and most preferably a alkaline or earth alkaline hydroxide) is added to the produced water to form alkalinized water, typically after a step of de -oiling and solids removal, wherein the base is added in an amount that increases solubility of the silica in the alkalinized water at least 25%, more typically at least 50%, even more typically at least 100%, and most typically at least 250% as compared to the produced water. Where desired, divalent cations are then removed from the treated water using an ion exchange resin to thereby soften the treated water. [0031] Likewise, it should also be appreciated that produced water comprising silica can be processed chemically breaking an emulsion in the produced water, removing oil and precipitate from the produced water, removing divalent cations from the produced water using an ion exchange resin, adding a base to the produced water to thereby form alkalinized water, and forming an intermediate quality steam product from the alkalinized water without a step of removing silica from the produced water. It is especially preferred that all process steps are performed without an active step of removing silicate from the produced water, the alkalinized water, and/or the softened treated water (e.g., via lime softening, precipitation, or other known manner). Such active steps are typically performed on the produced water to remove at least 10%, more typically 20%, even more typically at least 50%, and most typically at least 90% of the silica present in the produced water.

[0032] Contemplated treatment methods are especially remarkable as the feed- water quality for OTSGs, the most common form of boiler found in oilfield enhanced oil recovery projects and used in generating steam at pressures up to about 2,000 psig with treated produced waters was established twenty-five years ago and has changed little (SAGD facilities typically operate at lower steam pressures but often use the same quality criteria). Currently the accepted quality for many OTSG requires total hardness of equal or less than 0.5 mg/L (as CaC03), Silica equal or less than 50 mg/L, and oil equal or less than 10 mg/L. However, it should be appreciated that silica solubility increase at high pH and high temperature. Both conditions can be maintained in an OTSG, which in most cases produces about 75% steam and about 25% liquid. Table 1 illustrates solubility of silica in water at standard temperature (20 °C) and pressure (1 atm) as a function of pH.

pH mg/1 Si02

6-8 120

9 138

9.5 180

10 310

10.6 876

Table 1

[0033] Thus, and provided that the pH is suitably high, it should be noted that water entering the OTSG can contain significant quantities of silica, and indeed can even have higher quantities of silica per volume as the untreated produced water where silica is added. It is therefore contemplated that the water entering the steam generator will have at least 70% silica content, more typically at least 80% silica content, and most typically at least 90 % silica content as compared to the silica content of the untreated produced water. [0034] In further preferred aspects of the inventive subject matter, it should be appreciated that the inventor discovered that the alkalinized water can be used in a variety of ways after de-oiling, solids removal, and softening, and especially for steam generation. In particularly preferred aspects, the alkalinized water is directly used in a steam generator, despite the relatively high silica content as substantially all (i.e., typically at least 95%, more typically at least 98%, even more typically at least 99%, and most typically at least 99.9%) of the silica is dissolved in the softened treated water. Thus, the softened treated water may have a dissolved silica content of at least lOOmg/1, more typically at least 200 mg/1, even more typically at least 400 mg/1, and most typically at least 600 mg/1 at STP (standard temperature (20 °C) and pressure (1 arm)). Moreover, it should be recognized that the solubility is even further increased at elevated temperatures (up to 500 °F) and pressures (up to 2,000 psig) commonly encountered in steam generators, and particularly OTSGs.

[0035] As should be readily appreciated, use of the alkalinized water in steam generation may be performed in various manners, and all known manners are deemed suitable herein.

However, it is especially preferred that the alkalinized water is fed to a OTSG to so produce an intermediate quality steam product. Most commonly, the steam quality will be in the range of 60-90%), and in the majority of cases between 70-80%>. Where desired, the so produced intermediate quality steam product can be directly used for injection in EOR or SAGD operations, or may be processed by separation in a condensate separator. In such case, the isolated stream is a high quality steam product (i.e., at least 90%>, more typically at least 95% steam) that is then used for its intended purpose (e.g., injection in EOR or SAGD operations). The remaining condensate may be further processed to obtain additional water, for example, by use of an evaporator, brine concentrator, crystallizer, reverse osmosis or other filtration, electrolytic desalination, etc. The additional water may be used for further steam generation, disposition in a sewer system, or other use in a plant.

[0036] Therefore, it is contemplated that the quality of intermediate quality steam product may vary, and will typically be at least 60% quality, more typically at least 70% quality, and most typically 75-80 % quality. Likewise, the intermediate quality steam product may be at various pressures, and especially suitable pressures will typically be at least 500 psig, more typically at least 700 psig, even more typically at least 1000 psig, most typically at least 1500 psig. Consequently, the steam temperature is temperature at least 470 °F, more typically at least 505 °F, even more typically at least 546 °F, most typically at least 597 °F. The person of ordinary skill in the art will readily determine suitable pressures and temperatures, which will be at least in part be dictated by the type of hydrocarbon and the depth of formation. Thus, it is noted that at least a portion of the intermediate quality steam product and in other instances substantially all (i.e., at least 90%) of the intermediate quality steam product can be used for injection into the formation. One should further appreciate that the systems and methods presented herein may have applicability beyond EOR or SAGD processes. For example, contemplated systems and methods can be applied toward other waste water treatment processes, and especially desalter mudwash wastewater operations to reduce COD/BOD load on treatment plants.

[0037] In still further contemplated aspects of the inventive subject matter, the inventor noted that in most conventional operations, steam-water mixtures exiting the OTSG still often require steam quality improvement. Remarkably, it has not yet been appreciated in the art to eliminate the steam separator stage, silica removal equipment, and the large amount of chemicals required in utilizing high quality steam by using lower quality steam directly in SAGD reservoir operations as already described above.

[0038] Even more notably, using pre-treated lower quality steam will also eliminate concerns related to silica build-up in the interstitial region between injection and extraction wells as is explained in more detail below. Viewed from a different perspective, silica removal operations currently necessitate extensive capital equipment, maintenance and disposal costs, which can be avoided through the use of high silica wet steam mixture as a lower quality stream, delivering the same BTU content as is currently employed. Additionally the use of high silica steam-water injection results in the return of silica to the in-situ environment it originated from. Indeed, it should be appreciated that silica that is currently being removed and treated as a waste stream can now be re-injected into its source formation without adverse effects on the oil extraction operations. This re -injection of silica provides significant environmental and economic advantages through elimination of one of the major waste streams associated with SAGD operations.

[0039] More specifically, the inventor has discovered that the steam separator stage, silica removal equipment, and large amount of chemicals currently required in utilizing high silica make-up water can be eliminated by use of lower quality steam from pre-treated water streams directly in SAGD reservoir operations while eliminating concerns related to silica build up in the interstitial region between injection and extraction wells. [0040] During the initial phases of SAGD steam chamber expansion, oil flows from both the ceiling and slopes and its flow rate can be described by Butler's LinDrain equation:

[0041] During this phase flow rates are proportional to Vh where h reaches a maximum at the thickness of the reservoir (net pay thickness). This flow rate model is applicable until the steam chamber reaches the overlying cap rock at which point it spreads horizontally and oil flows become slope flows only and heat loss to the cap rock occurs. During this subsequent phase, the shape of the steam chamber can be approximated by a triangular cross section.

[0042] Total oil produced over the life of the facility is directly proportional to the area of the triangle which can be calculated as R x h, where R is the radial spread along the cap rock and h is the reservoir thickness. R is related to Θ, where Θ is the angle between the horizontal and the SAGD reservoir steam chamber. As Θ decreases, R grows, total oil produced grows and heat loss to the cap rock also grows. The area represented by the SAGD steam chamber can be written as h²/tan Θ. In typical SAGD reservoirs, the lower point in this inverted triangle can be treated as the producer well with the injector well located some distance S above the producer well. For simplicity the region between the two wells is treated as being filled with a bitumen/water mixture and the volume occupied is similarly described by the area of the triangle bounded by the SAGD reservoir and bitumen/water level. This area can be written as S²/tan Θ. The ratio of this bitumen/water volume to total produced oil volume is S²/h² and is constant throughout the entire reservoir development period. During initial reservoir development (until cap rock is reached), reservoir volumes developed will be less than what would be expected based in the height of the steam chamber.

[0043] Similar to total oil produced, silica production is directly related to the cross sectional area of the steam chamber along the axis of the injection well. As such, in situ silica (soluble fraction only), would be liberated from the produced portion of the reservoir and concentrated in the bitumen/water reservoir between the producer and injector wells. The degree of concentration is represented by the ratio of these two areas or S²/h² and actual silica levels in the bitumen/oil mixture will be a function of in situ levels (soluble fraction). The constant nature of this relative concentration is the reason that one does not see an increase in silica levels in produced water over time. Viewed from a different perspective, it should be appreciated that the silica deposition in the formation and dissolution from the formation are equilibrium processes that ultimately do not materially affect the silica content in produced water. Likewise, over-deposition of silica will not occur due to re-dissolution of the silica material from the formation.

[0044] For "thick" reservoirs one might expect total net pay thickness to be on the order of 33 meters and spacing between production and injection wells to be 5 meters, concentrations in the lower regions would be expected to be 43.56 times the initial in situ soluble

concentrations. Return of silica removed from produced water to the bitumen/water reservoir underground does not increase total silica in the underground reservoir.

[0045] Consequently, it should be recognized that in the pool is an equilibrium concentration that is a function of the pH and pool temperature, and material that can not stay in solution will precipitate out. It should be noted that this precipitation already happens with the current silica removal practice, since any reduction of silica below the solubility limit in the pool is immediately offset by dissolution of the precipitate back into the pool, sustaining pool silica concentrations at the saturation level consistent with pool chemistry and temperature (which is observed in operation). Produced water silica levels will not rise above this equilibrium level all other things being equal. Conversely, if the saturation level is not yet reached, pool chemistry is driven by the ratio laid out times the initial soluble silica concentration in the reservoir, and again the pool will reach an equilibrium level and the silica concentrations in produced water will remain unchanged. Thus, one operational concern should not relate to silica concentration in the circulating loop, but rather to the significance of the precipitate volume in the reservoir.

[0046] For example, in a case for a 1000 bpd facility operating for 5 years, about 250,000 metric tons of oil are removed plus some other amount of dissolved solids which are stripped out at the surface. Also displaced are 217 tons of silica, or about 0.1% of the removed oil. Using the reservoir dimensions above, the volume in the liquid region is about 2.5% of the total depleted volume which means 2.5% of the oil came from this volume. Consequently, it should be appreciated that the oil is replaced with a mass of silica that is 25 times smaller, ignoring the amount that is actually in solution in the loop. Some fraction of the silica in the pool will be in solution, further influenced by higher pH in those instances where higher pH steam-water flows from the OTSG are utilized, as well so the actual precipitate volumes will be even less.

[0047] Unless the context dictates the contrary, all ranges set forth herein should be interpreted as being inclusive of their endpoints and open-ended ranges should be interpreted to include commercially practical values. Similarly, all lists of values should be considered as inclusive of intermediate values unless the context indicates the contrary.

[0048] It should be apparent to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the scope of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms "comprises" and "comprising" should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced. Likewise, as used herein, and unless the context dictates otherwise, the term "coupled to" is intended to include both direct coupling (in which two elements that are coupled to each other contact each other) and indirect coupling (in which at least one additional element is located between the two elements). Therefore, the terms "coupled to" and "coupled with" are used synonymously. Where the specification claims refers to at least one of something selected from the group consisting of A, B, C .... and N, the text should be interpreted as requiring only one element from the group, not A plus N, or B plus N, etc.

Claims

claimed is:

A method of processing produced water, comprising:

providing a quantity of produced water, and de-oiling and removing solids from the produced water to thereby form treated water;

removing divalent cations from the treated water using an ion exchange resin to

thereby produce softened water;

increasing alkalinity of the softened water to thereby form alkalinized water, and forming an intermediate quality steam product from the alkalinized water; and using at least a portion of the intermediate quality steam product for injection into a formation.

The method of claim 1 wherein the step of using at least a portion of the intermediate quality steam product comprises separation of the intermediate quality steam product into a condensate and a high quality steam product, and a further step of injection of the high quality product into the formation.

The method of claim 2 further comprising a step of processing the condensate to obtain additional water, and using the additional water in the step of forming the intermediate quality steam product.

The method of claim 3 wherein the step of processing the condensate comprises feeding the condensate into a brine concentrator.

The method of claim 1 wherein the step of using at least a portion of the intermediate quality steam product comprises using substantially all of the intermediate quality steam product for injection into the formation.

The method of claim 1 wherein the step of forming the intermediate quality steam is performed using a once-through steam generator.

The method of claim 1 wherein the alkalinized water has a pH of at least pH 10.

The method of claim 1 wherein the intermediate quality steam product comprises between 10 and 30% condensate.

9. The method of claim 1 wherein the steps of increasing alkalinity, removing divalent cations, and forming the intermediate quality steam product are performed without a step of removing silicate from the produced water, the treated water, the softened water, and the alkalinized water, respectively.

10. The method of claim 1 further comprising a step of adding previously isolated silica to the produced water, the treated water, the softened water, or the alkalinized water.

11. The method of claim 1 wherein the ion exchange resin comprises a weak acid cation ion exchange resin.

12. A method of processing produced water, comprising:

providing produced water comprising silica, and de-oiling and removing solids from the produced water;

removing divalent cations from the produced water using an ion exchange resin; and adding a base to the produced water to thereby form alkalinized water, wherein the base is added in an amount that increases solubility of the silica in the alkalinized water at least 25% as compared to the produced water.

13. The method of claim 12 wherein the base is an alkaline metal hydroxide or an alkaline earth metal hydroxide.

14. The method of claim 12 wherein the base is added in an amount to achieve a pH of at least 10 in the alkalinized water.

15. The method of claim 12 wherein the step of de-oiling comprises a step of chemically breaking an emulsion in the produced water.

16. The method of claim 12 further comprising a step of forming an intermediate quality steam product from the alkalinized water, and still further comprising a step of using at least a portion of the intermediate quality steam product for injection into a formation.

17. A method of processing produced water, comprising:

providing produced water comprising silica;

chemically breaking an emulsion in the produced water, and removing oil and

precipitate from the produced water; removing divalent cations from the produced water using an ion exchange resin; adding a base to the produced water to thereby form alkalinized water, and forming an intermediate quality steam product from the alkalinized water without a step of removing silica from the produced water.

18. The method of claim 17 wherein the base is added in an amount to achieve a pH of at least 10 in the alkalinized water.

19. The method of claim 17 further comprising a step of adding previously isolated silica to the produced water or the alkalinized water.

20. The method of claim 17 further comprising a step of using at least a portion of the intermediate quality steam product for injection into a formation.